Medical Physicist's Notes · Neutron Monitor

Six monitor positions and two thresholds at each.

A neutron-monitor deployment is more than just one monitor at the vault door. A typical cyclotron-facility neutron field map needs monitors at six locations across vault, adjacent workplaces and corridor. Each monitor carries two threshold levels — warning and action. This post walks the placement logic, the threshold-setting rationale and what triggers a re-survey of the field map.

Monitor placement

Six locations in a typical cyclotron / PET facility

Location	Why	Priority
Vault interlock door (inside)	Reads the field operator faces at the moment of entry. Gates the safe-entry alarm.	Mandatory under AERB licensing
Vault interlock door (outside)	Reads the corridor field for personnel passing near the vault while production runs. Gates corridor evacuation if applicable.	Mandatory under AERB licensing
Adjacent radiopharmacy	Fast neutrons pass through standard hot-lab shielding. Documents staff workplace dose during runs.	Strongly recommended
Above the vault (next floor up)	Sky-shine neutrons can produce a measurable field on the floor above. Required when the floor above is occupied space.	Site-dependent
Cyclotron control room	Control-room neutron field is normally negligible but documented to confirm operator workplace compliance.	Documentation case
Target service / pneumatic transfer area	Target handling and pneumatic-transfer corridors carry activation neutrons during target service procedures.	Strongly recommended

Source: AERB Safety Code for Medical Cyclotron Facilities; IAEA Safety Reports Series 47.

Threshold setting

Warning vs action — the two-level architecture

Warning threshold: typically set at the workplace-dose-rate level that triggers an investigation but not an evacuation. Indicative value 5–10 µSv/h for cyclotron-adjacent workplaces; final value set against site-specific occupancy and the AERB licence.
Action threshold: typically set at the level that requires immediate area evacuation or interlock-driven beam-off. Indicative value 20–50 µSv/h; final value site-specific.
Vault-interior thresholds during beam-off (post-decay) are different — the vault is unoccupied during beam-on, so the vault-interior alarm is for the entry-decision window. Warning at the level acceptable for short-duration maintenance entry; action at the level that disallows entry.
Threshold-setting paperwork (site-survey + licensing rationale) goes into the AERB facility-licence dossier alongside the type-test certificate and the calibration record.

Source: AERB Atomic Energy (Radiation Protection) Rules 2004; ICRP Publication 75.

When to re-survey

Five triggers for a field-map refresh

Target service (chemistry change, target re-build): re-measure the post-beam-off decay curve at the vault-interior monitor. Activation-neutron tail may shift.
Shield modification (any structural change to the vault): full re-survey, including all interior and adjacent-workplace monitors. The change can move the field map significantly.
Beam-current uplift (if the cyclotron is upgraded to higher current): re-measure at all monitor positions. Workplace doses scale roughly linearly with average current.
Adjacent-workplace renovation (hot lab refit, ward construction): re-measure at the adjacent monitor and consider adding new monitor positions if workspace layout changes.
AERB inspection request: when the inspector asks to walk the field map, the monitor archive should already show the same map. Re-survey before inspection is a sign the map has been neglected.